Many types of liquid-level sensing devices have been disclosed in the prior art. Such devices, which are usually based on magnetic, electrical-conductivity or optical parameters, enable user manipulation of the positioning of the sensing elements for either facilitating the operation of the liquid level sensor itself or the issue of a sensible output at a desired liquid level in the monitored vessel, in accordance with one or more predetermined sensing elements. Thus, for example, U.S. Pat. Nos. 3,696,362 and 3,944,845 describe a conducting liquid level control devices, which indicate when the level of a conducting liquid reaches the predetermined minimum level, while U.S. Pat. No. 4,354,180 describes an optical liquid-level sensing probe. U.S. Pat. Nos. 3,849,771 and 4,771,272 describe a portable self-contained liquid level alarm devices having a housing in which a floating magnet actuates a reed switch. U.S. Pat. No. 3,702,910 describes a level sensing control device with a reed switch which can be rotateably adjusted to any position in respect of a magnetic member connected to a float member. An additional implementations of such magnetic devices have been described inter alia in U.S. Pat. Nos. 3,702,910; 3,823,328; 3,997,744; 3,978,299; 4,035,789; 4,066,858; 4,395,605; 4,458,118; 4,459,584; 4,499,348; 4,609,796; 4,748,299; 4,748,300; 5,103,673; 5,224,379; 5,426,271 and 5,562,003.
Most of the liquid-level monitoring and alarmed systems described in the prior art, including those based on a floating magnet, are not versatile and usually have drawbacks and/or are not convenient for user manipulation. It is specially seen in cases where the monitored vessel's interior is wholly inaccessible, this may occur in the case of a sealed vessel holding hazardous material or when the content and/or prevailing conditions do not allow a simple and convenient user access and manipulation. Similar difficulties exist in the case of heated vessels, and specially in the case of cooking pot or a like.
It often occurs in everyday cooking, that a heated liquid, e.g. soup, is left simmering on a heat source without supervision. It may occur that the cooking pot is forgotten on the heat source, whereby the victuals being cooked or the cooking pot itself may be ruined and, more severely, a hazardous situation may occur. There are known devices for automatically switching-off an electrical water heater when the water level drops below the level of the heating element of the heater. Also, U.S. Pat. No. 4,673,925 describes an alarm device for removably mounting onto the upper edge of a cooking pot for providing an alarm indication when the cooking pot's liquid contents begin to boil, whilst U.S. Pat. No. 5,229,751 describes a coffee pot including an integrally provided alarm device for providing a time based alarm indication in respect of the freshness state of its liquid contents. Also, U.S. Pat. No. 5,341,728 describes an alarm device which controls cooker and indicates end-of cooking timing.
In reality, only certain household containers, for example electric kettles, are provided with liquid-level sensors and automatic alarm and/or switch-off devices, while most cooking pots and pans do not include such devices. This is primarily because the liquid-level sensors and/or auto-switch devices of the art consume valuable space and disturb in cooking and/or cleaning. Moreover, even liquid-level sensing devices currently used for cooking devices don't contain a simple and convenient means allowing the user, whenever he desires, to easily change the level at which the sensor will activate the alarm.